designing an optimum percent cold work (%cw) for cold rolling of aluminum strips to achieve the best...

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Kingdom of Saudi Arabia

King Saud University

College Of Engineering

Mechanical Engineering Department

DESIGNING AN OPTIMUM PERCENT COLD WORK (%CW) FOR

COLD ROLLING OF ALUMINUM STRIPS TO ACHIEVE THE BEST

MECHANICAL PROPERTIES.

By

Fadhel Al Nakhli 426101760

Yousef Rikli 428114302

Supervised by:

Dr.Zain al Huda Shamsulhuda

Dr.Abdulhakim al Majid

A project submitted in partial fulfillment of requirement for the degree of Bachelor of Science in Mechanical Engineering in College of Engineering, King Saud University. 1432 – 1433 H (1) (2011-2012 G)

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We hereby approve the report entitled:

DESIGNING AN OPTIMUM PERCENT COLD WORK (%CW) FOR COLD ROLLING OF ALUMINUM

STRIPS TO ACHIEVE THE BEST MECHANICAL PROPERTIES.

Prepared by:

Fadhel Al Nakhli 426101760

Yousef Rikli 428114302

Principal advisor: _________________________

Signature _________________

Examiners: _________________________

Signature _________________

_________________________

Signature _________________

Date (2/1433 H) (1) (12/2011 G)

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Abstract

The cold rolled wrought aluminum products find wide industrial applications in

automotive, construction, and other engineering components. The degree of cold work

(%CW) has a significant influence on the mechanical properties of cold rolled metals.

We have designed a series of experiments involving various degree of with specified

parameters. We have used a Lab Rolling Mill to cold roll various samples of aluminum

strips. We will also use tensile testing machine, Vernier calipers, and cutting machines

during the project. We determined mechanical properties; and create graphical plots to

analyze the data. We conclude the project by recommending an optimum %CW for

aluminum strips to achieve the best mechanical properties.

Figure ‎0.1: experiment in process

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Contents Chapter 1 :INTRODUCTION ............................................................................................................. 4

1.1 Cold working of metal ........................................................................................................... 5

1.2 Aluminum and its forming ..................................................................................................... 6

1.3 Mechanical properties ........................................................................................................... 6

1.4 Project objectives .................................................................................................................. 7

Chapter 2: LITERATURE REVIEW ...................................................................................................... 8

2.1 Meaning of Design ................................................................................................................. 8

2.2 Design of a process ................................................................................................................ 9

2.3 Effect of Cold-Working in material ........................................................................................ 9

2.4 Design philosophy and process ........................................................................................... 11

2.4.1 Philosophy of design .....................................................................................................11

2.4.2 Some of the general concepts .......................................................................................12

Chapter 3: DESIGN PROCEDURE .................................................................................................... 13

3.1 Recognition of need ............................................................................................................ 13

3.2 Definition of problem .......................................................................................................... 14

3.3 Constraints ........................................................................................................................... 14

3.4Gathering of information ..................................................................................................... 14

3.4.1 Cold formed steel ..........................................................................................................15

3.4.2 Rolling ............................................................................................................................16

3.4.3 Aluminum ......................................................................................................................16

3.5 Conceptual design ............................................................................................................... 17

3.6 Detailed design .................................................................................................................... 18

3.7 Criteria for success .............................................................................................................. 18

Chapter 4: EXPERIMENTAL WORK ................................................................................................. 19

4.1 Material and methodology .................................................................................................. 19

CHAPTER 5: PROCESS-DESIGN MODELS AND DATA ANALYSIS ..................................................... 25

5.1Mechanical Behavior of each Design Model ........................................................................ 26

5.1.1 Design Model-1: ............................................................................................................26

5.1.2 Design Model-2 .............................................................................................................27

5.1.3Design Model-3 ..............................................................................................................28

5.1.4 Design Model-4 .............................................................................................................29

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5.2 Comparative Analysis of Design Models ............................................................................. 31

5.2.1 Strength comparisons ...................................................................................................31

5.2.2 Hardness comparisons ..................................................................................................32

5.2.3 Ductility comparisons ....................................................................................................33

Chapter 6: Conclusion ................................................................................................................... 34

References ..................................................................................................................................... 35

Appendex1......................................................................................... Error! Bookmark not defined.

Figures Figure ‎0.1: experiment in process Figure 1.1: Metal rolling process Figure 2.1: Effect of cold working on tensile and yield strength of one metal. Figure 2.2: Effect of cold working on tensile strength, hardness, ductility and grain size Figure 3.1: Flow chart of production of materials Figure 3.4.2: type of metal fabrication techniques Figure 3.4.3: chart describes the types of aluminum Figure 3.6: detailed design

Figure 4.1: chemical analysis

Figure 4.2: tensile test sample geometry (ASTM E8) Figure 4.3: stress strain diagram for specimen Figure 5.1.1: relationship between load and extension in % 10 CW Figure 5.1.2: relationship between load and extension in % 25 CW Figure 5.1.3: relationship between load and extension in % 35 CW Figure 5.1.4.1: relationship between load and extension in % 40 CW Figure 5.1.4.2: the Alligatoring failure phenomena that occurred in specimen 4 (%40 CW). Figure 5.2.1: ultimate strength for the different models Figure 5.2.2: Vicker’s hardness numbers for the different models Figure 5.2.3: %elongation for the different models

Tables Table 4.1: results for chemical analysis.

Table 4.4: as received (Specimen 0).

Table 5.1: Design models according to %CW

Table 5.1.1: Tensile test results for specimen 1(10%CW)




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Chapter 1 :INTRODUCTION

1.1 Cold working of metal

Cold working (CW) and recrystallization annealing are the fundamental phenomena of microstructure evolution in the processing of engineering materials. They are of major scientific interest and of great importance for a wide range of industrial applications. Recent and numerous studies carried out on aluminum alloy sheets have demonstrated that fatigue life of specimens containing a cold worked(CW) open hole can be improved by a factor of 4–20; this finding is of extreme technological significance in aerospace industries .( Burlat 2008 ) Deformation by cold rolling is a process by which the metal is introduced between rollers and then compressed and squeezed at room temperature (below the recrystallization temperature) for reducing its cross sectional area(Fig.1.1).

Figure 1.1: Metal rolling process

It is convenient to express the degree of plastic deformation as percent cold work as follows: % cold work = [(A0 - Ad)/ A0] x 100 (1.1)

% cold work = {

} (1.2)

Where A0 = the original area of the cross section, and Ad = the cross-sectional area after deformation.

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1.2 Aluminum and its forming

Aluminum and its alloys are characterized by a relatively low density (2.7 g/ as compared to 7.9 g/ for steel), high electrical and thermal conductivities, and a resistance to corrosion in some common environments, including the ambient atmosphere. Many of these alloys are easily formed by virtue of high ductility; this is evidenced by the thin aluminum foil sheet into which the relatively pure material may be rolled. Since aluminum has an FCC crystal structure, its ductility is retained even at very low temperature. The chief limitation of aluminum is its low melting temperature

[660 ], which restricts the maximum temperature at which it can e used. Forming operation are those in which shape of metal piece is changed by plastic deformation; for example, forging, rolling, extrusion, and drawing common forming techniques.

1.3 Mechanical properties

The follwoing mechanical Properties are important in engineering material (internet

source 2011B):

1. Yield point. If the stress is too large, the strain deviates from being proportional to the stress. The point at which this happens is the yield point because there the material yields, deforming permanently (plastically).

2. Yield stress. Hooke's law is not valid beyond the yield point. The stress at the yield point is called yield stress, and is an important measure of the mechanical properties of materials. In practice, the yield stress is chosen as that causing a permanent strain of 0.002.

The yield stress measures the resistance to plastic deformation.

The reason for plastic deformation, in normal materials, is not that the atomic bond is stretched beyond repair, but the motion of dislocations, which involves breaking and reforming bonds.

Plastic deformation is caused by the motion of dislocations.

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3. Tensile strength. When stress continues in the plastic regime, the stress-strain passes through a maximum, called the tensile strength TS), and then falls as the material starts to develop a neck and it finally breaks at the fracture point.

For structural applications, the yield stress is usually a more important property than the tensile strength, since once it is passed, the structure has deformed beyond acceptable limits.

4. Ductility. The ability to deform before braking. It is the opposite of brittleness. Ductility can be given either as percent maximum elongation maxor maximum area reduction.

%EL = max× 100 % (1.3)

%AR = (Ainitial–Afinal)/Ainitial(1.4)

Where %EL is the percent elongation and %AR is the percent area reduction.

These are measured after fracture (repositioning the two pieces back together).

6. Resilience. Capacity to absorb energy elastically. The energy per unit volume is the area under the strain-stress curve in the elastic region.

7. Toughness. Ability to absorb energy up to fracture. The energy per unit volume is the total area under the strain-stress curve. It is measured by an impact test.

1.4 Project objectives

1) To design optimum %CW for rolling of aluminums trips.

2) To study the effects of various %CW on hardness.

3) To study the effects of various %CW on yield strength and the tensile strength

4) To study the effects of various %CW on ductility (% elongation and % RA)

5) To comparatively analyze the rolling design models to select the best mechanical

properties for the cold worked aluminum strips.

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Chapter 2: LITERATURE REVIEW

2.1 Meaning of Design

Materials design is a branch of engineering design, with particular nuances related to the

hierarchical structure of materials and complex relations between process, structure, and

properties. Hence, it is useful to discuss in more general terms the conceptualization of

engineering design.

Research in engineering design is categorized into design philosophies, models, and methods.

Design theory is a collection of principles that are useful for explaining a design process and

provide a foundation for basic understanding required to propose useful methodologies. Design

theory explains what design is, whereas design methodology is a collection of procedures, tools,

and techniques for designers to use. Design methodology is prescriptive, while design theory is

descriptive [9–11]. Design methods have been developed from different viewpoints that

emphasize various facets of the overall design process. Some of these views, as summarized by

Evbuomwan and coauthors (Evbuomwan, Sivaloganathan et al. 1996), include (1) design as a

top-down and bottom-up process, (2) design as an incremental (evolutionary) activity, (3) design

as a knowledge-based exploratory activity, (4) design as an investigative (research) process, (5)

design as a creative (art) process, (6) design as a rational process, (7) design as a decision

making process, (8) design as an iterative process, and (9) design as an interactive process.

Although design methods are generally developed with a few of these viewpoints in mind, an

ideal design method should support all of these.

Pahl and Beitz (Pahl and Beitz 1996) identify four key phases that are common to any

prescriptive model for design. These phases include planning and clarification of task,

conceptual design, embodiment design, and detail design. Planning and clarification of task

involves identifying the requirements that the outcome of design should fulfill. These

requirements are then converted into a statement of the problem to be solved. Conceptual

design involves generation of principles used to satisfy the problem statement. Embodiment

design involves refinement of the solution for the purpose of eliminating those that are least

satisfactory until the final solution remains. During the detail design, all the details of the final

design are specified and manufacturing drawings and documentation are produced.

In contrast to the descriptive models of design, prescriptive models exemplify how design

should be done and not necessarily how it is done. Most of the prescriptive methods of design

are based on the assumption that any design activity consists of three core activities—analysis,

synthesis, and evaluation (ASE). Analysis is defined as the resolution of anything complex into its

elements and the study of these elements and of their relationships. Synthesis is the pulling

together of parts or elements to produce new effects and to demonstrate that these parts

create an order (Pahl and Beitz 1996). A general model of design can be visualized as a feedback

loop of synthesis, analysis, and evaluation.(David L. McDowell et al. 2010)

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2.2 Design of a process

Most engineering designs can be classified as- devices or system that are created by human

effort and did not exist before or are improvements over existing devices or system. Inventions,

or designs, do not suddenly appear from nowhere. They are result of bringing together

technologies to meet human needs or to solve problem. Sometimes a design is the result of

someone trying to task more quickly or efficiently. Design activity occurs over a period of time

and requires a step- by – step methodology.

We described engineers primarily as problem solvers. Design problems are open ended in

nature, which means they have more than one correct solution. The result or solution to a

design problem is a system that possesses specified properties.

Design problems are usually more vaguely defined than analysis problems.

Solving design problems is often an iterative process: As the solution to design problem evolves,

you find yourself continually refining the design.

While implementing the solution to a design problem. You may discover that the solution you've

developed is unsafe, too expensive, or will not work. You then “go back to the drawing board"

and modify the solution until it meets your requirements.

2.3 Effect of Cold-Working in material

A material is considered to be cold worked if its grains are in a distorted condition after plastic

deformation is completed. All the properties of a metal that are dependent on the lattice

structure are affected by plastic deformation or cold working. The following properties are

affected by cold work significantly:

1. Tensile Strength

2. Hardness

3. Yield Strength

4. Ductility

Tensile strength, yield strength and hardness are increased, while ductility is decreased.

Although both strength and hardness increase, the rate of change is not the same. Hardness

generally increases most rapidly in the first 10 percent reduction (cold work), whereas the

tensile strength increases more or less linearly. The yield strength increases more rapidly than

the tensile strength, so that, as the amount of plastic deformation is increased, the gap between

the yield and tensile strengths decreases. This is important in certain forming operations where

appreciable deformation is required. In drawing, for example, the load must be above the yield

point to obtain appreciable deformation but below the tensile strength to avoid failure. If the

gap is narrow, very close control of the load is required.

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Figure 2.3: Effect of cold working on tensile and yield strength of one metal.

Figure 2.4: Effect of cold working on tensile strength, hardness, ductility and grain size

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The increase in internal energy, particularly at the grain boundaries, makes the material more

susceptible to inter granular corrosion, thereby reducing its corrosion resistance. Known as

stress corrosion, this is an acceleration of corrosion in certain environments due to residual

stresses resulting from cold working. One of the ways to avoid stress corrosion cracking is to

relieve the internal stresses by suitable heat treatment after cold working and before placing

the material in service.

2.4 Design philosophy and process

According to an early NBS handbook on material testing (Natl. Bur 1913):

“An adequate measure of a given property is possible when (1) the property can be defined with sufficient exactness, (2) the material is of known composition or purity, (3) the attending conditions are standard or are known, (4) the experimental methods are theoretically correct, (5) the observations and their reductions are made with due care, and (6) the order of accuracy of the results is known. This ideal is rarely if ever reached, but as it is striven for the results pass from the qualitative to the quantitative stage and are called constants because redeterminations will not yield sensibly different results. Approximate results are improved upon steadily as more precise instruments and methods are devised. The degree of accuracy to be sought becomes a very practical matter in a testing laboratory. The time and labor involved in tests may well increase out of proportion as the limits of attainable accuracy are approached. For the determination of physical constants of fundamental properties of materials the degree of accuracy sought may be the maximum possible. In general, the degree of accuracy striven for should be that which is strictly good enough for the purpose at hand.”

2.4.1 Philosophy of design

The philosophy must contain a consistent set of principles within which contradiction does not

exist. It must possess an operational aspect. Or lead o action, because an exercise without

consequences is useless. Since its origin is empirical, it must contain a feedback mechanism

capable of evaluating the success with which principles are applied to particular situation and

for revealing short comings.

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2.4.2 Some of the general concepts

1) Need. Individual or social need must exist for the item.

2) Action. The material item must actually be realized.

3) Economic Utility. The item must be made available at a marketable price that includes profit

as well as cost.

4) Design procedure. Design proceeds iteratively from the abstract concept to the concrete

item.

5) Judicious Compromise. The basically empirical nature of all engineering dictates a constant

need for decision.

6) Evaluation. The many designs must be evaluated to establish the best alternative.

7) Optimality. The best alternative is optimized with respect to the most significant criterion.

8) Communication. To gain existence, the design must be communicated to the producer

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Chapter 3: DESIGN PROCEDURE

3.1 Recognition of need

Recognition of a need is the first and the most important step of the machine design or

system design cycle (Fig 3.1), without this first step no further steps of the machine

design can be taken. It is the need that gives birth to various other steps of the design. If

there is no need there won’t be any reasons to start the detailed, time consuming and

highly complex problem of designing. (S. K. Kataria & Sons1997 ‏)

Figure 3.1: Flow chart of production of materials

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Since strength of formed material is a requirement in the design of engineering

structures, it is very important to find the %CW which results in a high strength of the

material. This is why there is a need to design an optimum %CW which gives us the best

mechanical properties.

3.2 Definition of problem

Now we define the problem with reference to our project. In the cold rolling of Al strips,

there is a %CW which results in the poorest mechanical properties (e.g. strength), which

should be avoided.

3.3 Constraints

A. Hardness value of less than about 100 HBN so that not to damage the rolling

machine.

B. Elongation ratio must be greater than 25% to be able to produce many CW% of

significant differences.

C. Dimensions to fit the machine gap: width (150 mm).

3.4Gathering of information

Figure 3.4.1: type of metal fabrication techniques

Metal fabrication techniques

Miscellaneous casting forming

operation

forging rolling

Roll bending Roll forming Flat rolling Foil rolling Ring rolling

extrusion drawing

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3.4.1 Cold formed steel

When a steel member is cold-formed, it can be expected that the newly obtained

mechanical properties of the steel are different when compared to the original steel

used. The yield stress, ultimate tensile stress and ductility change. The cold forming

process also causes fundamental changes in the yielding type, whereby the initially

distinct sharp yield point present in most carbon steels is replaced by a rounded stress–

strain curve.

In structural engineering the change in mechanical properties as a result of cold working

has been studied earlier. Chajes et al. [1] conducted experiments on the influence of

uniform cold stretching on the mechanical properties of carbon steel sheeting. It was

observed that the increase in yield stress and ultimate tensile stress depend on the

direction of cold working and the properties of the original material. When the direction

of plastic stretching coincides with the direction of loading, increases in yield stress and

ultimate tensile stress were observed, underlining the potential benefit of cold working

steel in structural applications.

The influence of the manufacturing process on the mechanical properties of cold-

formed steel sections has also been studied extensively. Cold-formed sections are

produced from flat steel sheeting by means of various cold-forming techniques such

aspress-braking and roll forming. The influence on structural carbon steel has been

investigated experimentally by Karren [2], Karren and Winter [3], Key et al. [4] and

Abdel-Rahman and Sivakumaran [5] andon stainless steel by Rasmussen and Hancock

[6], Gardner and Nethercot [7] and Cruise and Gardner [8]. It was found that especially

the corner regions of these cold formed sections showed a significant increase in yield

stress and ultimate tensile stress. Karrensuggested that since the corner regions

represent up to 30% of the cross-sectional area, the influence of the altered mechanical

properties should be incorporated in structural calculations.

What is the difference between hot working and cold working?

When deformation is achieved at a temperature above that at which recrystallization

occurs, the process is termed hot working; otherwise it's cold working. With most of

forming techniques, both hot- and cold working procedures are possible. For hot-

working operation, large deformations are possible, which may be successively repeated

because the metal remains soft and ductile. Also, deformation energy requirements are

less than for cold working. However, most metals experience some surface oxidation,

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which results in material loss and a poor final surface finish. Cold working produce an

increase in strength with the attendant decrease in ductility, since the metal strain

hardness; advantages over hot working include a higher quality surface finish, better

mechanical properties and a greater variety of them, and closer dimensional control of

the finished piece. On occasion, the total deformation is accomplished in a series of

steps in which the piece is successively cold worked a small amount and then process

annealed. However, this is an expensive and inconvenient procedure.

3.4.2 Rolling

The most widely used deformation process consists of passing a piece of metal between

tow rolls; a reduction in thickness results from compressive stresses exerted by the roll.

Cold rolling may be used in the production of sheet, strip, and foil with high quality

surface finish. Circular shapes as well as I- beams and railroad rails are fabricated using

grooved.

3.4.3 Aluminum

The Aluminum Association (AA) has adopted a nomenclature similar to that of wrought

alloys. British Standard and DIN have different designations. In the AA system, the

second two digits reveal the minimum percentage of aluminum, e.g. 150.x corresponds

to a minimum of 99.50% aluminum. The digit after the decimal point takes a value of 0

or 1, denoting casting and ingot respectively (Arnold, 1995) .The main alloying elements

in the AA system are as follows:

1xxx series are minimum 99% aluminum

2xxx series copper

3xxx series silicon, copper and/or magnesium

4xxx series silicon

5xxx series magnesium

7xxx series zinc

8xxx series lithium

http://en.wikipedia.org/wiki/British_Standard

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Figure 3.4.2: chart describes the types of aluminum

3.5 Conceptual design

After recognizing the need and defining the problem, we thought how to solve the

problem for obtaining the optimum design for the cold rolling process. For this purpose

we decided to develop a flow chart for the detailed design which is discussed in the next

section (Sect. 3.6).

Metal alloy

Ferruos

Non ferruos

Copper and its alloys

Aluminum and its alloys

1xxx

2xxx

3xxx

4xxx

5xxx

6xxx

7xxx

8xxx

Magnesium and its alloys

Titanium and its Alloys

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3.6 Detailed design

Figure 3.6: detailed design

3.7 Criteria for success

1. Number of mechanical properties parameterized.

2. Time and money consumed by the project.

3. simplicity of procedure.

4. Number of design models experimentally produced.

5. Accuracy of results

Choosing and acquiring material

hardness and tesile tests

(1st verification)

chemical analysis (2nd verification)

cutting materail to

required dimensions

preforming cold rolling to required

%CW

measure %CW

(verification)

hardness and tensile tests

for workpiece rolled

Repeat for

new

specimen

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Chapter 4: EXPERIMENTAL WORK

4.1 Material and methodology

The Material was presumably Al 3003 aluminum alloy with no heat treatment, bought

from the local market, its dimensions were 120×20×2500 mm, the thickness 0f 20 mm

was chosen so that the thickness reduction after the rolling would be easily noticed and

measured, the width of 120 mm was accepted (rolling machine gap width =150mm).

The length was arbitrary taken as (L= 1550 mm), which was estimated to produce

enough specimens as we need. small piece from

the material was chemically analyzed using the

Scanning Electron Microscope (SEM) technique;

its results are shown in Table (4.1), the results fit

the Al 6061 more, so we assume the seller was

mistaken. It should be noted that due to a

peculiar similarity between silver and aluminum,

the microscope produced errors regarding the

percentage of silver in the specimen.

Table 4.2: results for chemical analysis.

Element Weight% Atomic%

Mg K 0.61 0.70

Al K 94.25 97.14

Si K 0.79 0.78

Mn K 0.69 0.35

Fe K 0.38 0.19

Ag L 3.28 0.84

Totals 100.00

The slab was brought to the university. A

workshop form was filled, accompanied with a

mechanical drawing of an ASTM E8 tensile test

specimen(Fig 4.2). The form was handed to the technicians at the Mechanical

Engineering Department workshop, who cut a slice from the slab and machined the

Figure 4.1: chemical analysis

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tensile test specimens. We conducted tensile testing by using computer controlled

universal tensile test machine to give ultimate tensile stress, yield stress, young modulus

and fracture stress. From the graphical plot (Fig 4.3) the percent elongation was

calculated, which turned out to be 30%, which is ductile enough for this project. A small

piece from the machined slice was ground, polished and then underwent a hardness

test by an electronic Vickers hardness test machine. The results were compiled in a table

(Table 4.4)

Figure 4.2: tensile test sample geometry (ASTM E8)

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Figure 4.3: stress strain diagram for specimen

Table 4.4: as received (Specimen 0).

In order to cold roll a number of samples, we cut the as-received slab to specimens of

dimension (250mm * 21.9mm * 20mm) (see figure 4.4).

%CW Model (1)

Tensile stress (UTS) 328 MPa Yield stress 90.7 MPa E (Young’s modulus) 5660 MPa % elongation 29.7% Hardness 91.15VHN (91.2 HBN)

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It is important for the safety of the rolling machine that we first allow a small

percentage thickness reduction (10%) when cold rolling. for this purpose, the following

calculations were made…(next page)

Figure (4.4): rolling specimen preparation

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To measure thickness before rolling ( ). we used a Vernier calipers;

Which was found to be ( :

L = 1370 mm

w = 120 mm

Area of cross-section before rolling =

=438

Specimen 1: %CWrequired = 10%

% CW = {

}

So 10 =

x 100

20- = 2

Hence, we adjusted distance between rolls as accurately as possible = 17.8 mm

Then performed rolling in two passes (t=19, 17.8) mm consecutively.

- Measurements after rolling:

then

%CWactual=

= 11%

Specimen 2

%CWrequired = 25%

% CW = {

}

so 25% =

x 100

20- = 2

Hence, we adjusted the gap between rolls = 15.2 mm

Then performed rolling in 4 passes: (t=19.35, 18.3, 17.2, and 15.2) mm consecutively.

- Measurement after rolling:

then

%CWactual=

= 23.5%

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Specimen 3

%CWrequired = 35%

% CW = {

}

so 35% =

x 100

20- = 5

Hence, we adjusted the gap between rolls = 12.8 mm

Then performed rolling in 6 passes.


then

%CWactual=

= 32.5%

Specimen 4

%CWrequired = 45%

% CW = {

}

So 45% =

x 100

18.4- = 8.28

Hence, we adjusted the gap between rolls =

Then performed rolling in 6 passes:


then

%CWactual=

= 38.59%

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CHAPTER 5: PROCESS-DESIGN MODELS AND DATA ANALYSIS

We here referred in sect. 3.6, about detailed design (see chapter 3) of this project. We

have also mentioned in Figure 3.4 about our repetition of experiments to establish

various design models. In this chapter, we have included results for each design model.

Table 5.1 present various design models. These design models would enable us to

obtain an optimum %CW design of cold rolling process to achieve the best mechanical

properties in the investigated 6061 aluminum alloy.

Table 5.1: Design models according to %CW

Design models %CW(aim) %CW(actual)

Design model-0 0 (as received) 0 (as received)

Design model-1 10 11

Design model-2 25 23.5



Design model-5 55 cancelled

The results for the Design Model-0 (as received material) have been presented in chapter 4.

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5.1Mechanical Behavior of each Design Model

5.1.1 Design Model-1:

As mentioned in table 5.1, the Design model-1 refers to 10% CW. The mechanical test results

data for this model are explained in the following sub-sections.

Hardness test results:

No. of indentations: 2

Average hardness value: 119.2 VHN

Tensile test results:

Figure 5.1.1: relationship between load and extension in % 10 CW


Maximum Load

Tensile stress at Maximum Load

Load at Yield (Offset 0.2 %)

Tensile stress at Yield (Offset 0.2 %)

Modulus (E-modulus)

Tensile extension at Break (Standard)

% Elongation

(N) (MPa) (N) (MPa) (GPa) (mm)

9660 367 3518 330.3 11 5 19.7

Comparison of Model-1 with Model-0:

Now we Compare Model-1 with Model-0, as regard to hardness. The value for

hardness has increased from 113VHN to 119.2VHN. That can be explained by the

strain hardening of the grains of the rolled strip. Tensile stress has also increased

from 328MPa to 367MPa. Meaning it approximately tripled in value.

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5.1.2 Design Model-2

As mentioned in table 5.1, the design model-2 refers to 25% CW. The mechanical test results

for this model are explained in the following sub-sections.







Maximum Load




Modulus (E-modulus)


% Elongation


10255 392 3668 344.4 12.8 4 15.4


Now we Compare Model-2 with Model-1, as regard to hardness, the value for

hardness has decreased from 119.2VHN to 122.5VHN. That might be a mistake in

specimen 2 hardness test. Tensile stress has also increased from 367MPa to

392MPa, which is a slight increase compared to that of model one.

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5.1.3Design Model-3









Maximum Load




Modulus (E-modulus)


% Elongation


10419 419 3285 308.5 12.8 4 14.7



hardness has increased from 122.5VHN to 126.4VHN. That can be explained by the

strain hardening of the grains of the rolled strip. Tensile stress has also increased

from 392MPa to 419MPa which is even a smaller increase than in model2..

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5.1.4 Design Model-4





Average hardness values: 87.7 VHN


Figure 5.1.4.1: relationship between load and extension in % 40 CW


Maximum Load




Modulus (E-modulus)


% Elongation


4100 223.6 2500 - - 4.8 18.9



hardness has decreased sharply from 126.4 VHN to 87.7 VHN. That can be

explained by the failure of the bar under this much CW and Alligatoring. Tensile

stress has also decreased from 419MPa to 223.6MPa, the trend here has reversed

due to the following failure …

Page | 30

Figure 5.1.4.2the Alligatoring failure phenomena that occurred in specimen 4 (%40 CW).

Page | 31

5.2 Comparative Analysis of Design Models

5.2.1 Strength comparisons

In the preceding section, we explained strength data obtained for each design model. In

this section we will compare the strengths obtained for the various design models.

Figure 5.2.1: ultimate strength for the different models

Figure (5.2.1) illustrates the trend in the ultimate tensile strength (UTS) for the design

models 0-4. A look on the bar chart shows that the UTS first increases gradually from

328MPa (for Model0) (as received material) to 419MPa (for model3) (35%CW). Then

UTS decreases from 419MPa to 223.6MPa (for model 4).

We know from material science that as cold work percentage increases, so does the

strain hardening associated with it. The strain hardening effect is represented by the

increase in the ultimate tensile strength as we move from model -0 to model-3.

The drop in UTS as we move from model-3 to model-4 is justified as follows. The model-

4(45%CW specimen) had fractured during rolling resulting in Alligatoring, when a

specimen is split due to Alligatoring, the strain hardening from the cold work is not

distributed evenly vertically in the bar’s cross section, resulting in some regions strain

hardening more than others, some maybe not at all, if the hardness and tensile

specimens were extracted from those regions, the sudden drop in strength (UTS); and

hence the low UTS value for the design model 4 would be reasonable.

The bar chart of Figure (5.2.1) shows that the highest UTS value is for model-3 (35%),

suggesting that it is the best design model.

328 367

392 419

223.6

model 0model 1model 2model 3model 4

UTS (MPa)

UTS (MPa)

Page | 32

5.2.2 Hardness comparisons

Figure 5.2.2: Vicker’s hardness numbers for the different models

Figure (5.2.2) illustrates the trend in the hardness for the design models 0-4. A look on

the bar chart shows that the hardness increases gradually from 113VHN (for Model0) (as

received material) to 126.4VHN (for model3) (35%CW). Then hardness decreases from

126VHN to 78.7 VHN (for model 4).

We know from material science that as cold work percentage increases, so does the

strain hardening associated with it. The strain hardening effect is represented by the

increase in the hardness as we move from model -0 to model-3.

The drop in hardness as we move from model-3 to model-4 is justified as follows. The

model-4(45%CW specimen) had fractured during rolling resulting in Alligatoring, when a




specimens were extracted from those regions, the sudden drop in hardness (VHN); and

hence the low VHN value for the design model 4 would be reasonable.

The bar chart of Figure (5.2.2) shows that the highest VHN value is for model-3 (35%),

suggesting that it is the best design model.

113.4 119.2 122.5 126.4

87.7


hardness (VHN)

hardness (VHN)

Page | 33

5.2.3 Ductility comparisons

Figure 5.2.3: %elongation for the different models

Figure (5.2.3) illustrates the trend in the Ductility for the design models 0-4. A look on

the bar chart shows that the ductility first decreases from 29.7% (for Model0) (as

received material) to 19.7% (for model1) (10%CW). Then it decreases from 19.7% to

15.4% (for model 2).It then increases again to 14.7% (for model3).Then it increases from

14.7% to 18.9% (for model 4).

As it is known the ductility is highest for non-worked materials (specimen 0), and as the

cold work increases through specimens 1-3, so does the ductility

The increase in %EL as we move from model-3 to model-4 is justified as follows. The

model-4(45%CW specimen) had fractured during rolling resulting in Alligatoring, when a




specimens were extracted from those regions, the sudden increase in ductility (%EL) and

hence the high %EL value for the design model 4 would be reasonable.

The bar chart of Figure (5.2.3) shows that the highest %EL value is for model-0 (as

received), suggesting that it is the best design model.

29.7

19.7

15.4 14.7

18.9


%EL (ASTM E8)

%EL(ASTM E8)

Page | 34

Chapter 6: Conclusion

A single trend can be observed in the specimens with regard to all mechanical

properties under study; two things can be said about that trend;

1- It is proportional to the percent cold work.

2- It is reversed after the failure at specimen4 (45%CW)

Both tensile strength and hardness increase with %CW, and ductility decreases. Both

reach an extreme point at specimen3 (35%) before reversing at failure.

From strength point of view, model 3 seems to be the best design model. However, this

model results in limited ductility which is a drawback. However, considering the

industrial applications of Aluminum, strength is always more important, hence design

model3 is suggested.

Page | 35

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Page | 37

Appendices

Attached are two documents:

1- Gantt chart.

2- Chemical analysis for model0 (as received material).

designing an optimum percent cold work (%cw) for cold rolling of aluminum strips to achieve the best...

Documents

degree of cold work

effect of cold

best mechanical properties

optimum percent cold

cold working of metal

design philosophy

philosophy of design

cold formed steel